It is known in complementary metal-oxide-semiconductor, CMOS, photonic integrated circuits to use single polarisation grating couplers, SPGC, to couple optical signals of a preselected polarisation, transverse electric, TE, mode or transverse magnetic, TM, mode into a planar silicon photonic waveguide, typically from an optical fibre. An SPGC-TE or SPGC-TM is a passive CMOS silicon photonically enabled device which typically comprises 3 major functional parts; a one-dimensional diffractive grating followed by a taper and a single-mode silicon photonic waveguide. A typical design following the silicon on insulate (SOI) concept consists of a silicon based substrate, a silicon core layer having a refractive index of about 3.474, a thin SiOx over-cladding provided on top of the silicon core, having a thickness of a few hundred nm and a refractive index of about 1.50, a thick SiO2 under-cladding provided under the silicon core layer, having a thickness of a few μm and the refractive index of about 1.444, which is often called the buried oxide (BOX), and a total internal reflection mirror between the under-cladding and the substrate. In order to support TE-mode propagation, a planar waveguide should in general have a width which is larger than its height, typically by a factor of 2. To support TM-mode propagation a planar waveguide should have a width which is much smaller than its height, typically also a factor of 2, as set out in from “Silicon Photonics II, Components and Integration”, edited by David J. Lockwood and Lorenzo Pavesi, 2011.